In an evolved packet system (EPS) network, a serving GPRS support node (SGSN) or mobility management entity (MME) connected to an S-GW through an S4 interface is referred to as S4 SGSN/MME.
In a procedure that a user equipment (UE) moves in the EPS network, a packet data networks gateway (P-GW), as an anchoring point of user data in the EPS network, does not change, and the S-GW may change as the location of the UE is moved. In a procedure that the UE moves from a general packet radio service (GPRS)/universal mobile telecommunications system (UMTS) network to the EPS network, a gateway GPRS support node (GGSN) and the P-GW, as anchoring points of the user data in the GPRS/UMTS network and the EPS network, do not change, and the UE uses no S-GW in the GPRS/UMTS network, and needs to use a serving gateway (S-GW) after moving to the EPS network. Under the two situations, the new-side mobility management node (S4 SGSN/MME) may increase transmission time delay of user service data after selecting a new-side S-GW for the UE.